The most frequently PHA produced is poly(3-hydroxybutyrate) or PHB [2]. The ability to produce PHB has been correlated with improved survival under stress conditions or in competitive environments [5, 6]. PHB is generally produced in conditions of carbon oversupply and low levels of other nutrients such as nitrogen, phosphate or oxygen [7]. The biosynthesis of PHB is dependent on the activity of the following enzymes: (i) a 3-ketothiolase which condenses two acetyl-CoA yielding acetoacetyl-CoA (encoded by phbA), (ii) a NADPH-dependent acetoacetyl-CoA

reductase which reduces acetoacetyl-CoA to (R)-3-hydroxybutyryl-CoA AZD8931 (encoded by phbB) and (iii) the PHB synthase (encoded by phbC) that catalyses the polymerization of (R)-3-hydroxybutyryl-CoA to form the polymer [8, 9]. This polymer is stored intracellularly as insoluble inclusion bodies called PHB granules [1] which also contain about 2% protein as well as phospholipids [10]. The main protein associated with the PHB granules is phasin (encoded

by phaP) which prevents coalescence of buy AZD2171 PHB granules by coating the granule surfaces [11–14]. However, other proteins have also been found associated with the granules, including transcriptional regulators such as PhaF from Pseudomonas oleovorans GPo1, PhaR from Paracoccus denitrificans, and PhaR from Ralstonia eutropha H16 [15–17]. Expression of enzymes involved in PHA/PHB biosynthesis and the granule-associated phasin are reported to be regulated at the transcriptional level [15, 16, 18–26]. This regulation may include repressors as well as find more activators [21]. The proteins PhbR from Azotobacter vinelandii UW136 [22] and PhaD from Pseudomonas putida KT2442 [24] are transcription activators. In contrast, PhaR of P. denitrificans represses phaR expression by

binding to a TGC rich region which overlaps the -35/-10 promoter [16]. In R. eutropha H16 the PhaR protein binds to the -35/-10 phaP promoter at two sites: the transcriptional start site and upstream from the -35 at the promoter region, thereby blocking RNA polymerase [17]. The PhaR binding site determined in R. eutropha comprises two 12 bp O-methylated flavonoid repeated sequences not related to those observed in P. denitrificans, suggesting that DNA-binding sites for PhaR recognition and the mechanisms of regulation may vary. The β-Proteobacterium Herbaspirillum seropedicae SmR1 is a plant-endophytic diazotroph found in association with economically important graminaceous species such as sugar cane, sorghum, rice and maize [27]. H. seropedicae SmR1 has been already described as a PHB producer using glucose as carbon source [28], however the molecular aspects of its PHB metabolism have not been addressed. The H.